1. Field of the Invention
The present invention relates to an amplifier circuit employing MOSFETs, and in particular, to a circuit for use with a high frequency amplifier circuit employing a MOSFET for compensating for a secular change in a drain current of a MOSFET.
2. Description of the Related Art
A drain current IDQ of a MOSFET at the time of no signal input thereto is determined by the magnitude of a gate-source voltage VGS thereof. FIG. 4 shows a VGS versus IDQ characteristic of such a MOSFET. In general, the electric characteristics such as an operational point of a MOSFET vary depending upon the magnitude of a drain current thereof, so with an amplifier employing a MOSFET, the drain current IDQ of the MOSFET is set so as to provide an optimal electrical operational point.
However, a MOSFET has a so-called xe2x80x9chot carrierxe2x80x9d phenomenon that its drain current IDQ changes over time. FIG. 5 illustrates an example of such a secular change in the drain current of a MOSFET with a constant gate-source voltage VGS thereof. As clearly seen from FIG. 5, as the drain current IDQ of the MOSFET changes, the electrical operational point of the MOSFET varies, thus posing a problem that the characteristics of the amplifier having an optimal electrical operational point at the time of the manufacture thereof change over time.
In view of this, the present invention is intended to obviate the above-mentioned problem of the prior art of a change in the operation point of an amplifier due to a change in a drain current of a MOSFET at the time of no signal input, and has its object to provide a MOSFET amplifier circuit which includes a compensation circuit comprising an inexpensive MOSFET and a constant current circuit for making the drain current of the MOSFET constant to thereby maintain the operational point of the amplifier at an optimal state at all the times.
Bearing the above object in mind, according to the present invention, there is provided a MOSFET amplifier circuit comprising: a first MOSFET; a constant current circuit for generating a current of a constant magnitude; a second MOSFET having a constant drain current controlled by said constant current circuit; and a voltage control circuit for applying a gate-source voltage of said second MOSFET to a gate-source voltage of said first MOSFET.
With the above arrangement, the voltage control circuit detects, for example, a gate-source voltage of the second MOSFET, amplifies and supplies it to the first MOSFET as a gate-source voltage thereof, whereby a voltage based on a secular change in the gate-source voltage of the second MOSFET, which provides a constant drain current for the second MOSFET, is given to the first MOSFET as its gate-source voltage, thus reducing a change in the drain current of the first MOSFET or making it at a constant value.
In this regard, it is to be noted that the voltage control circuit has a voltage amplification factor set in such a manner that the detected gate-source voltage of the second MOSFET is amplified in accordance with a difference in magnitude between the secular changes in the first and second MOSFETs, and supplied to the first MOSFET as its gate-source voltage. As a result, according to this invention, employing an inexpensive MOSFET for the second MOSFET, a MOSFET amplifier circuit capable of compensating for the drain current of the first MOSFET can be provided at a low cost.
In a preferred embodiment of the invention, the voltage control circuit comprises operational amplifiers and a plurality of resistors, and the constant current circuit comprises an operational amplifier, a variable resistor, resistors and a power supply.
Preferably, the second MOSFET is formed in a chip into which the first MOSFET is built.
With this arrangement, the secular change characteristics of the first and second MOSFETs can be made substantially the same, thus rendering circuit design easy and simple.
The above and other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description of preferred embodiments of the invention taken in conjunction with the accompanying drawings.